A semiconductor laser is used as a light source for recording and reproduction in an optical information recording/reproducing apparatus. Semiconductor lasers have a large variation in light emitting output when there is a change in ambient temperature. In general, the threshold current I.sub.th for the light output of a semiconductor laser varies as shown by the following equation in accordance with a change in temperature. EQU I.sub.th =(T.sub.j).alpha.exp (T.sub.j /T.sub.O)
T.sub.0 =100.about.150.degree. K., PA0 T.sub.j (.degree. K.) is the junction temperature of the semiconductor laser.
Therefore, the output light from the semiconductor laser is ordinarily monitored by a photo sensitive device such as a pin diode or the like and is fed back to a drive circuit of the semiconductor laser and a control system is constituted, thereby regulating the output light of the semiconductor laser. In optical information recording/reproducing apparatuses, it is necessary in the recording mode to modulate the output light of the semiconductor laser by a recording signal. However, the level of the output light for recording is a few times larger than the level of the output light for reproduction and also the output light for recording is modulated at a high frequency, so that it is difficult to constitute the control system by feeding back the monitored signal for recording. Therefore, the output light is not modulated by the recording signal, but the voltage of the control system obtained with the output light DC level for reproduction is held and a predetermined output light level which is needed for recording is further added to this held voltage thereby controlling and regulating the output light to stabilize it upon recording.
FIG. 1 shows a practical example. A reference numeral 1 denotes a semiconductor laser; 2 is a pin diode to monitor the output light of the semiconductor laser; 3 an operational amplifier to generate a control voltage for the control system; 4a and 4b transistors to modulate the current caused to flow through the semiconductor laser 1 by a recording signal 5; 6 a variable resistor for allowing a reference voltage to be generated to set the output light for reproduction; and 7a to 7d analog gate switches. Only the switches 7a and 7b are turned on in the reproduction mode, while only the switches 7c and 7d are turned on in the recording mode. Switching between recording and reproduction is performed by controlling the ON/OFF of the analog switches 7a to 7d by a recording gate signal 8. The monitor signal detected by the pin diode 2 is compared with the reference voltage upon reproduction and the control voltage is outputted by the operational amplifier 3. This control voltage is inputted to a sample-hold circuit 9 only for reproduction. Since the analog gate switches 7a and 7b are turned off for recording, the control system for feedback of the monitor signal is disabled. On one hand, the analog gate switches 7c and 7d are turned on and the semiconductor laser is driven using the control voltage which was sampled and held by the sample-hold circuit 9 immediately before the recording. Further, a current source which is constituted by a transistor 10 is enabled to secure the output light level which is needed for recording
FIG. 2 shows the relation between the forward current flowing through the semiconductor laser 1 in the arrangement of FIG. 1 and the output light level. P.sub.P indicates the reproducing power level and P.sub.R represents the recording power level. T denotes the ambient temperature of the semiconductor laser. .circle.1 is the current value controlled by the feed back of the monitor signal. Even if the ambient temperature changes to T' and the forward current versus output-light level characteristic changes, the current value is controlled to .circle.1 '. Therefore, the reproducing power is maintained at a constant level of P.sub.P. Upon recording, the current value of .circle.2 is added to by the transistor 10 thereby assuring the recording power level P.sub.R. The current value of .circle.2 to be added is always constant. The above-mentioned light output control method needs a premise such that although the threshold current I.sub.th largely varies due to a change in ambient temperature, the gradient of the forward current versus output-light level characteristic, namely, K=.DELTA.P.sub.W /.DELTA.I is always constant for the temperature change. However, the gradient K actually largely changes over a wide temperature range so that it cannot be ignored and further the gradient K varies as well due to the time-dependent change of the semiconductor laser.
FIG. 2 shows an operation mode such that when the ambient temperature changes to T', the gradient K changes and the output light level for recording can not be stably controlled. Specifically speaking, the forward current becomes .circle.1 ' with the change of threshold current I.sub.th ' and the output light level P.sub.P is secured for reproduction; however, a current value .circle.2 ' to be added for recording is constant irrespective of the ambient temperature and the current value of .circle.2 is also equal. Therefore, the recording power changes from P.sub.R to P.sub.R ' and the output light cannot be stably controlled for recording. It is necessary to make the recording bit length small to increase the recording density on the optical disk. In order to secure the signal quality in the reproduced signal of such small recording bits, very stable recording power is required because the signal quality largely depends upon the recording power.
As described above, when considering a wide temperature range and a time-dependent change of the semiconductor laser as well, the semiconductor laser drive circuit which has been conventionally proposed cannot stably control the output light for recording.
The present invention intends to solve the problems in the foregoing conventional example and it is an object of the invention to provide a semiconductor laser drive circuit having a simple arrangement for controlling the output light level so that it remains constant even in the recording mode of modulating the output light of a semiconductor laser by a recording signal.